To enable inexpensive power amplifiers (PA) in wireless communication devices or user equipment (UE), hereinafter collectively referred to as user equipment, the communication system may allow the option of using signals with small amplitude variations. In such cases, the linear region of the PA can be smaller or non-existent.
In Single-Carrier Frequency Division Multiple Access (SC-FDMA) systems such as Long-Term Evolution (LTE) or NarrowBand Internet of Things (NB-IoT), a single-subcarrier, or single-tone, transmission is used to achieve a signal with close to unit amplitude. SC-FDMA has two subcarrier spacing options: 15 kHz for single-tone and multi-tone transmissions, and 3.75 kHz for single-tone transmission only. The objective of NB-IoT is to address improved indoor coverage, support for massive number of low throughput devices, low delay sensitivity, ultra-low device cost, low device power consumption and optimized network architecture. In one or more embodiments, two modulation options are considered for NB-IoT uplink which are pi/2 BPSK and pi/4 QPSK. In these predefined modulations, the constellation is rotated pi/2 or pi/4 radians every symbol. In general, these rotations would allow smoother transitions between constellation points, reducing the peak to average power ratio (PAPR). These modulation options are considered for uplink data and control channel transmissions, except for M-PRACH preambles.
The NB-IoT uplink signal, in one or more embodiments, is composed of one to twelve 15 kHz subcarriers within the 180 kHz bandwidth. The signal spectrum characteristics 10 of a twelve subcarrier or twelve tone transmission is illustrated in FIG. 1. The transmitted signals have a maximum power level of 23 dBm power. In GSM, UEs have at least 33 dBm maximum output power, and the interference is controlled by ensuring that the transmission is contained within a spectral emission mask set according to the maximum transmit power of the UEs. Since NB-IoT UEs will typically transmit with 23 dBm output power, two special emission mask are used in the evaluations. The “33 dBm GSM mask 14” corresponds to the mask requirements that a GSM UE would fulfill, and the “23 dBm mask 16” where the 33 dBm mask has been adjusted down by 10 decibels. FIG. 1 illustrates a twelve tone NB-IoT uplink transmission under ideal conditions. As illustrated in FIG. 1, the twelve tone NB-IoT transmission fulfills the GSM spectrum masks 14 and 16.
FIG. 2 illustrates signal spectrum characteristics 10 of a single-subcarrier transmission of NB-IoT with 15 kHz subcarrier spacing, positioned in the leftmost, middle and rightmost, subcarrier under ideal conditions. In other words, FIG. 2 illustrates the power spectral density 12 of a single 15 kHz subcarrier NB-IoT uplink transmission 18 at subcarrier offsets 0, 5 and 11. As illustrated in FIG. 2, the figure shows that the 15 kHz NB-IoT uplink transmission 18 fulfills GSM masks 14 and 16 also with single-subcarrier transmission, where the figure represents both BPSK and QPSK modulation. FIG. 3 illustrates the signal spectrum characteristics 10 of multi-tone NB-IoT with 15 kHz spacing. In particular, NB-IoT with 15 kHz subcarrier spacing fulfills the GSM spectral emission masks 14 and 16 requirements under ideal conditions such as using an ideal power amplifier.
In wireless communication, single-carrier signals can have unit amplitude, but the amplitude may vary at the transition between modulation symbols. Such a situation is problematic. A way to minimize the amplitude variation between modulation symbols such as Binary Phase-Shift Key (BPSK) modulation symbols is to rotate each subsequent symbol constellation by 90 degrees, or pi/2 radians, creating a “pi/2 BPSK modulation”. For Quadrature Phase Shift Keying (QPSK) the corresponding rotation is 45 degrees or pi/4 radians, creating pi/4 QPSK. By minimizing amplitude variations between modulation symbols the average power ratio is reduced in the transmission waveform.
FIG. 4 is a diagram of the signal spectrum characteristics 10 of a pi/2 BPSK single-subcarrier NB-IoT transmission at different subcarrier positions. In particular, FIGS. 4 and 5 described herein use a PA model with impairments that mimic a realistic PA model as an ideal (unrealistic) PA model would likely not have issues with meeting a GSM spectral emission mask (GSM mask) requirements, discussed below, but would likely fail in the field or real life. As illustrated in FIG. 4, the power spectral density 12 of pi/2 BPSK fulfills the 33 dBm GSM spectrum mask 14 requirements at all subcarrier positions but is not able to fulfill the 23 dBm mask 16 requirements at all subcarrier positions.
FIG. 5 is a diagram of signal spectrum characteristics 10 of a pi/2 BPSK single-subcarrier NB-IoT transmission 18 at different subcarrier positions with a 2.3 dB back-off applied. As illustrated in FIG. 5, the power spectral density 12 of pi/2 BPSK transmission 18 fulfills the 33 dBm GSM spectrum mask 14 requirements at all subcarrier positions and also fulfills the 23 dBm mask 16 requirements at all subcarrier positions, but requires back-off such that the maximum output power level of the PA is disadvantageously reduced. PA backoff also results in a reduction in PA efficiency and lower energy efficiency, which reduces the battery life.